The Senate’s farm bill cloture vote Thursday morning poses a critical test for the Agriculture Committee leadership, which needs a strong showing to clear the way for passage Monday and begin to heal the breach sparked by revisions in the commodity title.

Center stage is Chairwoman Debbie Stabenow, who finds herself cast as the older sister charged with getting her siblings to finish their chores. But beneath her calm — few are more adept at smiling and talking at the same time — the Michigan Democrat knows her time is running short.

Impatient to move onto immigration reform, Majority Leader Harry Reid (D-Nev.) needs to have the decks cleared by early next week, and any Senate stall on the farm bill will be felt across the Capitol.

All signs there still indicate that the week of June 17 is being set aside for what promises to be a stormy floor debate. House Speaker John Boehner (R-Ohio) is becoming more engaged — pursuing his own campaign against a new dairy program. And House Agriculture Committee Chairman Frank Lucas (R-Okla.) appeared encouraged after a meeting with his conference Tuesday morning.

That said, a dangerous drift has settled in this week in the Senate. Bad blood between Sens. Pat Toomey (R-Pa.) and Mary Landrieu (D-La.) has made Stabenow’s job harder in trying to reach agreement on a finite list of amendments. The floor situation has been greatly complicated by the death of Sen. Frank Lautenberg (D-N.J.) and subsequent services in New York on Wednesday and the Capitol on Thursday.

But going forward, the most enduring challenge for Stabenow may be the regional and ideological divide, which cost her precious Southern votes last June and now, could mean the loss of well-placed allies from the Midwest.

Stabenow’s former partner, Sen. Pat Roberts (R-Kan.), appears to be off the reservation. More important are Sens. John Thune (R-S.D.), chairman of the Republican Conference, and Mike Johanns (R-Neb.), who was Agriculture Secretary under President George W. Bush.

Behind this split is a decade of change in which net farm income in the Midwest had increased much faster than the national average while income for the Southeast and Southern Plains has trailed behind — or even declined.

The Corn Belt’s immense success has fortified free market advocates like Thune. The South’s experience has had the opposite result: convincing Senate Republicans from that region — and Lucas in the House — that a more traditional safety net is still required.

The Senate bill still tilts heavily to the Midwest, devoting as much as $23.7 billion over 10 years to a new Agricultural Risk Coverage program, which is especially favorable to corn. But to help the South, Stabenow and her new ranking Republican, Mississippi Sen. Thad Cochran, have added a modest $3.4 billion “adverse market” countercyclical program including target prices for rice and peanuts.

The Agriculture Department chief economist Joseph Glauber told POLITICO that the chances are low that ARC or the adverse market program will violate limits set by the World Trade Organization. But this hasn’t cooled the rhetoric from Midwest Republicans, who have been predicting trade wars overseas and planting distortions at home.

Roberts has been most acerbic: “The WTO stove is hot. We should not reach out to touch it again.”

But Thune also has picked up the mantle and given his leadership position a greater potential impact. In an interview, he conceded that part of his strategy is aimed more at future negotiations with the House, but he was also reluctant to commit to the cloture vote Thursday.

“The real difference is one is based on market. The other is based on Congress — that’s the old approach to doing things,” Thune told POLITICO. “That’s where the House is going to be, and the House target prices are significantly higher than the target prices in the Senate. So if we don’t have a position going into Conference with the House that takes a more market-oriented approach we’re going to end up with a bill that doesn’t have any reform in it.

“The House target prices are set at pretty high levels,” Thune said. “They have the potential is be very much trade distorting, planting distorting and get us back to where farmers are planting for the government.”

Johanns shares the same antipathy toward target prices and joined Roberts and Thune in opposing the Senate bill in committee. He said Wednesday that he had yet to make up his mind on the cloture vote but was also candid to admit that what Stabenow and Cochran have proposed in target prices is relatively modest.

“In the grand scheme of this farm bill, the target price piece of this is a pretty small piece. And I certainly acknowledge that,” Johanns told POLITICO. “Every day I ask, `Would you vote against the whole farm bill in the end because of the target price issue?’ And the conclusion I come to is: `Look, I want a farm bill. We’ll be better off with a farm bill than no farm bill. …I want to be a yes vote on a farm bill.’”

Indeed Southern Republicans, pointing to the Congressional Budget Office’s price projections and scoring of the Senate bill, would argue that the Midwest’s free market stance is neither pure nor free.

The ARC program is costly, since the Senate requires only a 12 percent shortfall in prices to trigger some payment versus the 15 percent drop set in the House.

There are many variables: increased crop yields can reduce the government’s exposure significantly. But working backward from CBO price projections, ARC could pay out as much as 16 cents to 21 cents per bushel in the early years. And just as with target prices, farmers will pretty much know the benchmark for ARC payments before they plant.

“I’m not really a big fan of ARC to be honest,” Johanns said. “ARC just came along, and it appeared in the bill. I appreciate that it has some benefit for Nebraska; I understand that. But my farmers aren’t asking for that. They just say don’t mess up crop insurance. It works. We understand that.”